In-Circuit Transistor Tester

In Circuit Transistor Tester Schematic

Here is a circuit that can indicate the condition of a transistor by using two LEDs. A good NPN transistor conducts during the positive half cycle (pulses are generated by 556 dual timer) and D5 is off while D6 flashes. With a good PNP transistor D6 is off and D5 Flashes.

PARTS LIST

R1

470kΩ

R2

470Ω

R3

470Ω

C1

0.47µF (Electrolytic)

D1

1N4148

D2

1N4148

D3

1N4148

D4

1N4148

D5

RED LED

D6

GREEN LED

IC1

NE556

Both LEDs flash if the transistor is open and will be off if it is shorted.

The tester checks the transistor even if in-circuit resistance across the transistor is as low as 50 ohms; below this the glow becomes dim. Diodes in the ensure that only low saturation voltage of a good transistor turns the LED off.

Have very carefully built this too but no joy at all.
Suspect I need to ground unused inputs or something but am not sure, also suspect that voltage rating on electrolytic may be important as it powered up lighting both LEDs dimly after some minutes with a 0.47uF 50V cap which was the lowest value I had on hand.
Anyone actually had it going and happy with it? If so what did you do with unused inputs exactly.
Just want to build a basic transistor checker that works reliably